This is interesting, so I'll take a stab at it. There seem to be several fouls at work here, but most of them are minor.
1. Two friends playing together with strangers - did they disclose this fact? If not, they should have.
2. Unbreakable alliance - possibly forged before the game. If this was the case, probably guilty of meta-gaming, if not in the strictest sense. This, btw, is the reason I dislike playing with groups of friends -- even my own.
3. Player A logged in as Player B. Here's where it gets a bit sticky. IF Player B had asked a neutral third party player to log in for him and make the moves, I assume it would have been fine. So, the issue is that Player A did it instead of anybody else. Here's my question. What if Player A had NOT done those moves as asked? Win at all costs, right?
But he did enter the moves correctly and honestly, so I guess that, really, there's no foul here ... right? I'm confusing myself a bit on this point.
I guess if it was me, and I was a moderator, I would post on the global chat exactly what you have discovered here, but there was no clear rule-breaking going on. Also, to warn A & B that if they want to play together again, they would be required to announce that they are friends in RL. Hard to enforce perhaps, but at least give them the warning.
And also maybe have them rethink the whole unbreakable alliance thing. It's makes things less fun for everybody involved.